Modern electrophotography with reusble photoconductors involves either flash, optical, scanning or electronic scanning of endless belt or drum photoconductors. Electronic scanning of photoconductive drums provides certain advantages, including ease of cross track and skew registration of color images and image enhancement advantages associated with electronic manipulation of the signal controlling the scanning operation.
As electronic exposure systems develop higher resolutions and good electrophotographic toners of extremely small size are developed, continuously higher quality multicolor and single color imaging is possible. However, as such high quality is approached, mechanical deviations from perfection are much more noticeable.
For example, any deviation in the motion of a photoconductive drum while it is being exposed by a laser or comparable exposure device can show up as an image defect. The noticeability of the defect will, of course, be much more pronounced if the overall system itself is of extremely high quality. If the system is a color system in which multiple images are combined to form a multicolor image, an error in placement of one portion of one image can show up in the final image as a noticeable change in color.
U.S. Pat. Nos. 4,546,060 to E. T. Miskinis and T. A. Jadwin; 4,473,029 to G. F. Kasper, A. S. Kroll and M. Mosehauer; and 4,531,832 to A. S. Kroll and F. A. Shuster, describe a magnetic brush development approach that can be used with extremely fine toners for highest quality imaging, for example, high quality color imaging. In this approach, a magnetic core having magnetic poles which alternate circumferentially around the core periphery is rapidly rotated in close proximity to a development zone. A non-magnetic sleeve around the core supports a developer of hard magnetic carrier particles and insulative toner. The rapid rotation of the core causes rapid pole transistions through the development zone and through portions of the outside of the sleeve leading up to the development zone. These pole transistions cause the hard magnetic carrier particles to rotate or "flip" in a direction generally causing them to move around the sleeve in a direction opposite to that of the rotating core.
This system is presently being used commercially to provide high quality color images utilizing an endless belt photoconductor.
U.S. Pat. Nos. 4,139,296; 4,170,287; 4,067,296; 4,550,068 and 3,643,311, all show cylindrical members which have magnetic poles in a helical shape around the cylinder. In general, such cylinders are used for moving magnetic particles in both the circumferential direction and in an axial direction to provide good mixing of the particles as the cylinder is rotated.